Powering circuit with alternative and/or concurrent power sources

I think the answer to that question is "yes": e.g. Panasonic - Nickel Metal Hydride Batteries - Technical Handbook 2000 explicitly says that you can't trickle charge NiMH, except for a defined period of a few hours after rapid charging. With NiMH you need to carefully measure the voltage while charging to look for the "end of charge" condition. Having a circuit run directly off the battery will also make that harder.

You could however do:

• on plugging in, start charge
• stop charging as normal (voltage dip detection) and set a timer
• after approx 24 hours, retrigger the charging cycle

Or select a different battery chemistry which can be float charged such as NiCad.

The circuit looks OK as far as it goes, but I have two observations.

a) Is the material capable of polarisation? If so, then uni-directional current will steadily change the apparent conductivity over reading time. Examples would be damp earth for instance, or skin under gel electrodes.

A mitigation for this would be either to make your current source switchable polarity, or make the analogue multiplexers switchable between ground and current source. While full AC would work, it would complicate detection by your DAQ, and short periods of DC excitation, reversed between readings to provide zero average current, may work well enough.

b) If your multiplexers are switchable between GND and the current source, and you use a bipolar current source, then you need only 1:8 multiplexers, not 1:16, which may be a worthwhile hardware or tracking saving. Connect the outputs of the two multiplexers alternately to the electrodes, and you still have full control over which pair of electrodes receives which polarity of excitation.